Internally illuminated signs are used for many purposes. Some internally illuminated signs show only simple text. Examples of such signs are signs that display words such as “EXIT” OR “OPEN”. Other signs, especially signs used in advertising, will typically include graphical images as well as text. Typically, such signs are illuminated by fluorescent tubes inside the signs. It is important that such signs have uniform illumination. It has been discovered that the ratio of the distance between the center of one tube and the center of the adjacent tube to the distance from the center of any of the tubes to the front surface of the sign should be no greater than 2:1. When this ratio is greater the regions directly in front of the tubes will be perceived by a viewer as being brighter than other regions of the sign.
An alternative type of sign when text is to be displayed is a neon sign. Neon signs are advantageous because they may be made bright with very high contrast so that they catch the attention of the viewer. A disadvantage of neon lighting, however, is that it is fragile and easily broken. A further disadvantage of neon is that each neon tube must be made of custom blown glass. Another disadvantage of neon lighting is that it requires high operating voltages.
Fluorescent and neon light sources can be categorized as linear light sources. Yet another type of sign is lit using point light sources. One type of point light source rapidly being adopted by the sign industry is the LED. LEDs have numerous advantages over the latter two types. They are low voltage, consume less power, are more robust and demonstrate very long life requiring fewer bulb changes. When using such point sources of light, however, it is especially difficult to maintain proper illumination uniformity.